Circuit arrangement for operating discharge lamps

ABSTRACT

A DC-AC converter in a half bridge configuration is used for operation with high pressure discharge lamps as well as low pressure discharge lamps. The converter consists in two switches (T 1 -T 2 ) serially connected through a first series of diodes (D 1 , D 2 ) and a second series of diodes (D 3 ,D 4 ) to which at their middle points (N 1 , N 2 ) a first and a second lamp loads (L 1 ,L 2 ,LA 1 ,LA 2 ) are connected. Furthermore, the second terminals of the lamp loads are connected to the middle point of a series of capacitors (C 3 ,C 4 ) which are also connected to the terminals (Ki,K 2 ) of the supply voltage source. The converter includes also four diodes (D 5 ,D 6 ,D 7 ,D 8 ) which shunt the switches (T 1 ,T 2 ) and their respective series of diodes (D 1 ,D 2 ,D 3 ,D 4 ). The switches (T 1 ,T 2 ) are controlled by a controller (CC) which renders alternatively conductive at low frequency the two switches (Ti,T 2 ). In the first operating state, the first switch (T 1 ) is rendered conductive and non-conductive at a high-frequency while the second switch (T 2 ) is maintained in a non-conductive state, in the second operating state the second switch (T 2 ) is rendered conductive and non-conductive at high-frequency while the first switch (T 1 ) is maintained in a non-conductive state. The series of two capacitors (C 3 ,C 4 ) can be replaced by a series of two switches (T 3 ,T 4 ) in order to obtain a full bridge configuration of the converter. The particular configuration of the network and its operating mode allow to drive the lamp loads in such a way that the difference in power consumed by the lamps is comparatively small.

FIELD OF THE INVENTION

The invention relates to a circuit arrangement for operating a lamp loadcomprising

-   -   input terminals for connection to a supply voltage source,    -   a series arrangement I comprising a first switching element and        a second switching element and connecting the input terminals,    -   a control circuit coupled to respective control electrodes of        the first switching element and the second switching element for        controlling the conductive state of the first and the second        switching element,    -   a series arrangement II comprising a first circuit element A and        a second circuit element B and connecting the input terminals,    -   a first load circuit comprising a first ballast inductor and        first lamp connection terminals and connecting a terminal N1 of        the series arrangement I between the switching elements to a        terminal of the series arrangement II between the first circuit        element A and the second circuit element B.

BACKGROUND OF THE INVENTION

Such a circuit arrangement is a DC-AC-converter of the bridge type andis often used for the operation of both high pressure discharge lamps aswell as low pressure discharge lamps. The circuit arrangement can be afull bridge or a half bridge. A half bridge comprises only two switchingelements, since the first circuit element A and the second circuitelement B are both formed by capacitors. A full bridge, however,comprises four switching elements, since the first circuit element A andthe second circuit element B are formed by a third and fourth switchingelement. In a full bridge the control circuit is more complicated thanin a half bridge since the conductive state of four switching elementsinstead of only two switching elements needs to be controlled. Duringoperation the amplitude of the voltage across the load circuit(s) in afull bridge is twice as high as it is in a half bridge powered by thesame supply voltage source. As a consequence a full bridge can be usedto operate two lamps in series so that each of the lamps carries thesame current. In case the lamps have approximately the same lampvoltage, as is usually the case for low pressure discharge lamps, thelamps will approximately consume the same amount of power and haveapproximately the same light output. In case of high pressure dischargelamps, however, the lamp voltage depends strongly on the age of the lamp(age=number of burning hours). As a consequence the light outputs of twohigh pressure lamps operated in series can differ substantially when thelamps have a different age. The voltage across the load circuit of ahalf bridge powered by the same supply voltage source will generally notbe high enough to operate such a series arrangement of lamps. Adisadvantage of the full bridge is that, because of the two additionalswitches and the more complicated control circuit, it is more expensivethan a half bridge. As an alternative for of operating two lamps inseries in a fall bridge, two load circuits comprising a lamp and aballast inductor each, can be connected in parallel in a half bridge. Incase the lamps are low pressure mercury lamps that are operated with ahigh frequency lamp current, the lamp currents can be controlled atsubstantially equal values by making use of an equalizer transformer.However, such a transformer is an expensive component. In case the lampsare high pressure lamps operated by a low frequency AC current, anequalizer transformer cannot be used to control the lamp currents sincethe low frequency would demand a very big transformer. Nevertheless, incase no equalizer transformer is used, a difference in lamp voltagebetween the lamps will in practice often result in substantialdifferences in the amounts of power supplied to the lamps.

SUMMARY OF THE INVENTION

The invention aims to provide a circuit arrangement that is capable ofoperating two lamps in parallel in such a way that the difference inpower consumed by the lamps is comparatively small.

A circuit arrangement as described in the opening paragraph is thereforecharacterized in that the circuit arrangement comprises

-   -   a series arrangement III comprising terminal N1, a first diode        D1 and a second diode D2 and connecting the first switching        element and the second switching element, terminal N1 being        situated between first diode D1 and second diode D2,    -   a series arrangement IV comprising a third diode D3 and a fourth        diode D4 and connecting the first switching element and the        second switching element,    -   a fifth diode D5 shunting the first switching element and diode        D1,    -   a sixth diode D6 shunting the first switching element and diode        D3,    -   a seventh diode D7 shunting the second switching element and        diode D2,    -   an eighth diode D8 shunting the second switching element and        diode D4,    -   a second load circuit comprising a second ballast inductor and        second lamp connection terminals and connecting a terminal N2 of        the series arrangement IV, situated between the third diode D3        and the fourth diode D4 to a terminal of the series arrangement        II between the first circuit element A and the second circuit        element B.

When a conducting switching element in a circuit arrangement accordingto the invention is rendered non-conductive the ballast conductors inthe first and second load circuit cause the currents in the loadcircuits to maintain their polarity for a short time lapse, while theiramplitude is decreasing. During this time lapse the load circuitcurrents are conducted by part of the diodes D5–D8, forming “free wheeldiodes”. In a circuit arrangement according to the invention the currentin each of the load circuits, immediately after a switching element hasbeen rendered non-conductive, is flowing through a different “free wheeldiode”. As a consequence the two lamps are operated independently. Ithas been found that because of this independent operation a differencein lamp voltage, does not cause a substantial difference between theamounts of power consumed by the lamps.

It be remarked at this stage that a circuit arrangement according to theinvention can be rendered suitable for the operation of more than twolamps in parallel. For each additional lamp four more diodes and anadditional load circuit have to be implemented in the circuitarrangement. For instance operation of three lamps in parallel can berealized as follows. The circuit arrangement is equipped with a furtherseries arrangement V connecting the first and the second switchingelement and comprising two further diodes. Two additional diodes eachshunt a further diode and one of the switching elements and a third loadcircuit comprising a third ballast inductor and third lamp connectionterminals connects a terminal between the further diodes to a terminalbetween the first circuit element A and the second circuit element B.

Good results have been obtained for embodiments of a circuit arrangementaccording to the invention, wherein the first circuit element A and thesecond circuit element B each comprise a capacitor. Such embodiments arehalf bridge circuits, that are comparatively cheap. It has been foundmore in particular that such embodiments are very suitable for operatinghigh pressure discharge lamps in parallel in case the control circuit isequipped with means for alternately at a low frequency operating thecircuit arrangement in a first and a second operating state, wherein inthe first operating state the first switching element is renderedconductive and non-conductive at a high frequency while the secondswitching element is maintained in a non-conductive state, and whereinin the second operating state the second switching element is renderedconductive and non-conductive at a high frequency while the firstswitching element is maintained in a non-conductive state. This mode ofoperation is commonly referred to as “commutating forward”. Preferablythe circuit arrangement is equipped with a power control loop forcontrolling the average value of the total power consumed by both lampsat a desired value by adjusting the time lapse during which the firstswitching element is rendered conductive during each high frequencyperiod in the first operating state and during which the secondswitching element is rendered conductive during each high frequencyperiod in the second operating state. Such a power control loopcompensates for tolerances in the components of the circuit arrangement.

Good results have also been obtained for embodiments of a circuitarrangement according to the invention, wherein the first circuitelement comprises a third switching element and the second circuitelement comprises a fourth switching element, the control circuit beingcoupled to respective control electrodes of the third switching elementand the fourth switching element for controlling the conductive state ofthe third and the fourth switching element. Such embodiments are fullbridge circuits that allow the independent operation of lamps with ahigh lamp voltage in parallel or alternatively the operation of two ormore series arrangements of lamps (with a similar lamp voltage) inparallel. Also for these embodiments of a circuit arrangement accordingto the invention it has been found that they are particularly suitablefor operating high pressure discharge lamps in parallel in case thecontrol circuit is equipped with means for alternately at a lowfrequency operating the circuit arrangement in a first or-a secondoperating state, wherein in the first operating state the second and thethird switching elements are maintained non-conductive while the fourthswitching element is maintained conductive and the first switchingelement is rendered conductive and non-conductive at a high frequency,and wherein in the second operating state the third switching element ismaintained conductive while the second switching element is renderedconductive and non-conductive at a high frequency and the first and thefourth switching elements are maintained non-conductive. Also in theseembodiments it is desirable that the circuit arrangement is equippedwith a power control loop to compensate for tolerances in the componentsof the circuit arrangement.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of a circuit arrangement according to the present inventionwill be further explained making reference to a drawing. In the drawing:

FIG. 1 shows a first embodiment of a circuit arrangement according tothe invention with two lamps connected to it, and

FIG. 2 shows a second embodiment of a circuit arrangement according tothe invention with two lamps connected to it.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, K1 and K2 are input terminals for connection to a supplyvoltage source. Input terminals K1 and K2 are connected by means of aseries arrangement of first switching element T1, diode D1, terminal N1,diode D2 and switching element T2. Diodes D1 and D2 respectively form afirst and a second diode. Diodes D1 and D2 together with terminal N1form a series arrangement III connecting the first and the secondswitching element. Series arrangement III is shunted by a seriesarrangement of diode D3, terminal N2 and diode D4. Diodes D3 and D4respectively form a third and a fourth diode. Diodes D3 and D4 togetherwith terminal N2 form a series arrangement IV connecting the first andthe second switching element. The first switching element T1, the secondswitching element T2 and the parallel arrangement of series arrangementsIII and IV together form a series arrangement I. The series arrangementof diode D1 and first switching element T1 is shunted by diode D5 thatforms a fifth diode. The series arrangement of diode D3 and firstswitching element T1 is shunted by diode D6 that forms a sixth diode.The series arrangement of diode D2 and second switching element T2 isshunted by diode D7 that forms a seventh diode. The series arrangementof diode D4 and second switching element T2 is shunted by diode D8 thatforms an eighth diode. Respective control electrodes of the first andthe second switching elements are coupled to output terminals of acircuit part CC. In FIG. 1 this coupling is indicated by means of dottedlines. Circuit part CC is a control circuit for controlling theconductive state of the first and second switching element. Inputterminals of control circuit CC are also coupled to the diodes D5–D8 toenable the control circuit CC to monitor which of the diodes D5–D8 are(is) carrying a current and which diodes are non-conducting. Thiscoupling has not been indicated in FIG. 1, to avoid FIG. 1 becomingunclear because of a plurality of dotted lines. Input terminals K1 andK2 are also connected by means of a series arrangement of capacitors C3and C4. Capacitors C3 and C4 respectively form a first circuit element Aand a second circuit element B. Terminal N1 is connected to a commonterminal of capacitors C3 and C4 by means of a series arrangement of afirst ballast inductor L1, first lamp connection terminal K3, highpressure discharge lamp LA1 and first lamp connection terminal K4. Thefirst lamp connection terminals K3 and K4 are connected by means of afilter capacitor C1. First ballast inductor L1, first lamp connectionterminals K3 and K4, high pressure discharge lamp LA1 and filtercapacitor C1 together form a first load circuit. Terminal N2 isconnected to a common terminal of capacitors C3 and C4 by means of aseries arrangement of a second ballast inductor L2, second lampconnection terminal K5, high pressure discharge lamp LA2 and second lampconnection terminal K6. The second lamp connection terminals K5 and K6are connected by means of a filter capacitor C2. Second ballast inductorL2, second lamp connection terminals K5 and K6, high pressure dischargelamp LA2 and filter capacitor C2 together form a second load circuit.

The operation of the circuit arrangement shown in FIG. 1 is as follows.In case input terminals K1 and K2 are connected to a supply voltagesource supplying a DC supply voltage, the control circuit CC alternatelyand at a low frequency operates the circuit arrangement in a first and asecond operating state. During the first operating state the firstswitching element T1 is rendered conductive and non-conductive at a highfrequency, while the second switching element T2 is maintained in anon-conductive state. When the first switching element T1 is conductivethe current through the first ballast inductor increases linearly withthe time. Similarly the current through the second ballast inductor alsoincreases linearly with the time, However, since LA1 and LA2 are bothhigh pressure lamps their lamp voltages may differ considerably forinstance because of a difference in age (age=total number of burninghours). In case for instance lamp LA1 has a much lower lamp voltage thanlamp LA2, this causes the current through the first ballast inductor L1to increase faster than the current through the second ballast inductorL2. As a consequence, the amplitude of the current through first ballastinductor L1 has a higher amplitude than the current through the secondballast inductor L2, when the first switching element is renderednon-conductive. After the first switching element T1 has been renderednon-conductive, diode D7 conducts the current through the first ballastinductor L1 and diode D8 conducts the current through the second ballastinductor L2. The polarity of the currents through the first and thesecond ballast inductors L1 and L2 is maintained, but their amplitudesdecrease linearly. Since the amplitude of the current through the secondballast inductor was smaller than that of the current through the firstballast inductor, at the time the first switching element T1 wasrendered non-conductive, the amplitude of the current through the secondballast inductor will drop to zero first and diode D8 will becomenon-conductive while diode D7 still conducts. Although some dampenedringing will take place due to the presence of parasitic capacitances,the amplitude of the current through the second ballast inductor issubstantially maintained equal to zero until the current through thefirst ballast inductor reaches zero too so that also diode D7 stopsconducting. When the currents through diode D7 and diode D8 have bothdropped to zero, the first switching element is rendered conductive oncemore. This can be done immediately after both diodes have becomenon-conductive (this mode of operation is called critical discontinuousmode) or a predetermined time lapse after both diodes have becomenon-conductive (this mode of operation is called discontinuous mode).

Since in the circuit arrangement shown in FIG. 1 the time intervalneeded for the current through the first ballast inductor L1 to drop tozero may be (and in practice nearly always will be) different from thetime interval that is needed for the current through the second ballastinductor L2 to drop to zero, the operation of high pressure lamp LA1 isindependent from the operation of high pressure lamp LA2. It has beenfound that this independent operation of the two lamps causes theamounts of power consumed by the lamps to differ comparatively little,even when the lamps have substantially different lamp voltages becauseof a different age (=different number of burning hours). The current ineach of the ballast inductors has a triangular shape. By means of thefilter capacitors C1 and C2, these triangularly shaped currents aretransformed into continuous DC currents that flow through the lamps LA1and LA2.

During the second operating state, the first switching element ismaintained in a non-conductive state, while the second switching elementis rendered conductive and non-conductive at a high frequency. When thesecond switching element is non-conductive, diodes D5 and D6 carry thecurrent through respectively the first ballast inductor L1 and thesecond ballast inductor L2. Only after diode D5 and diode D6 have bothbecome non-conductive, second switching element T2 is renderedconductive again, either immediately or after a predetermined timelapse. Also in this second operating state, the current in each of theballast inductors has a triangular shape. By means of the filtercapacitors C1 and C2, these triangularly shaped currents are transformedinto continuous DC currents that flow through the lamps LA1 and LA2. Thepolarity of the currents through the lamps and the ballast inductors isreversed with respect to the first operating state.

In FIG. 2, circuit parts and components that correspond to similarcircuit parts and components in the embodiment shown in FIG. 1 arelabeled with the same references. The configuration of the embodimentshown in FIG. 2 only differs from the embodiment shown in FIG. 1, inthat the capacitors C3 and C4 have been replaced by a third switchingelement T3 and a fourth switching element T4 respectively. In theembodiment shown in FIG. 2, third switching element T3 and fourthswitching element T4 form a first circuit element A and a second circuitelement A respectively. Third switching element T3 and fourth switchingelement T4 together form a series arrangement II connecting the inputterminals. Respective control electrodes of the third switching elementT3 and the fourth switching element T4 are coupled to output terminalsof the control circuit CC. In FIG. 2 this coupling is indicated by meansof dotted lines. Respective input terminals of control circuit CC arecoupled to diodes D5–D8 to monitor which diodes are conducting a currentand which diodes are non-conducting. Also in FIG. 2 these couplings arenot indicated.

The operation of the circuit arrangement shown in FIG. 2 is as follows.In case input terminals K1 and K2 are connected to a supply voltagesource supplying a DC supply voltage, the control circuit CC alternatelyand at a low frequency operates the circuit arrangement in a first and asecond operating state.

In the first operating state switching elements T2 and T3 are maintainednon-conductive while switching element T4 is maintained conductive andswitching element T1 is rendered conductive and non-conductive at a highfrequency. In the second operating state switching element T3 ismaintained conductive while switching element T2 is rendered conductiveand non-conductive at a high frequency and switching elements T1 and T4are maintained non-conductive. Since the circuit arrangement shown inFIG. 2 is a full bridge circuit, the voltage that is present duringoperation over the load circuits is twice as high as the voltage presentover the load circuits in the half bridge circuit shown in FIG. 1.Otherwise, the operation of the embodiment shown in FIG. 2 is completelyanalogous to the operation of the circuit arrangement shown in FIG. 1.During each of the two operating states the circuit arrangement isoperated in either the critical discontinuous mode or the discontinuousmode, resulting in triangularly shaped currents flowing through theballast inductors that are transformed into continuous DC lamp currentsby means of the filter capacitors C1 and C2. Also in this embodimentindependent lamp operation is assured in the first operating state byrendering switching element T1 only conductive after both diodes D7 andD8 have become non-conductive and in the second operating state byrendering switching element T2 only conductive after both diodes D5 andD6 have become non-conductive.

It be remarked that in practice it is desirable, for the embodimentshown in FIG. 1 as well as for the embodiment in FIG. 2 to install apower control loop for controlling the average value of the total powerconsumed by both lamps at a desired value by adjusting the time lapseduring which the first switching element is rendered conductive duringeach high frequency period in the first operating state and during whichthe second switching element is rendered conductive during each highfrequency period in the second operating state. Such a power controlloop as such is very well known in the art. The power control loopassures that the total amount of power consumed by different practicalembodiments of the same circuit arrangement will approximately be thesame although the electrical properties of similar components used inthe different practical embodiments may differ.

1. Circuit arrangement for operating a lamp load comprising inputterminals for connection to a supply voltage source, a seriesarrangement I comprising a first switching element and a secondswitching element and connecting the input terminals, a control circuitcoupled to respective control electrodes of the first switching elementand the second switching element for controlling the conductive state ofthe first and the second switching element, a series arrangement IIcomprising a first circuit element A and a second circuit element B andconnecting the input terminals, a first load circuit comprising a firstballast inductor and first lamp connection terminals and connecting aterminal N1 of the series arrangement I between the switching elementsto a terminal of the series arrangement II between the first circuitelement A and the second circuit element B, characterized in that thecircuit arrangement comprises a series arrangement III comprisingterminal N1, a first diode D1 and a second diode D2 and connecting thefirst switching element and the second switching element, terminal N1being situated between first diode D1 and second diode D2, a seriesarrangement IV comprising a third diode D3 and a fourth diode D4 andconnecting the first switching element and the second switching element,a fifth diode D5 shunting the first switching element and diode D1, asixth diode D6 shunting the first switching element and diode D3, aseventh diode D7 shunting the second switching element and diode D2, aneighth diode D8 shunting the second switching element and diode D4, asecond load circuit comprising a second ballast inductor and second lampconnection terminals and connecting a terminal N2 of the seriesarrangement IV, situated between the third diode D3 and the fourth diodeD4 to a terminal of the series arrangement II between the first circuitelement A and the second circuit element B.
 2. Circuit arrangementaccording to claim 1, wherein the first circuit element A and the secondcircuit element B each comprise a capacitor.
 3. Circuit arrangementaccording to claim 1, wherein the first circuit element comprises athird switching element and the second circuit element comprises afourth switching element, the control circuit being coupled torespective control electrodes of the third switching element and thefourth switching element for controlling the conductive state of thethird and the fourth switching element.
 4. Circuit arrangement asclaimed in claim 2, wherein the control circuit is equipped with meansfor alternately at a low frequency operating the circuit arrangement ina first and a second operating state, wherein in the first operatingstate the first switching element is rendered conductive andnon-conductive at a high frequency while the second switching element ismaintained in a non-conductive state, and wherein in the secondoperating state the second switching element is rendered conductive andnon-conductive at a high frequency while the first switching element ismaintained in a non-conductive state.
 5. Circuit arrangement as claimedin claim 3, wherein the control circuit is equipped with means foralternately at a low frequency operating the circuit arrangement in afirst or a second operating state, wherein in the first operating statethe second and the third switching elements are maintainednon-conductive while the fourth switching element is maintainedconductive and the first switching element is rendered conductive andnon-conductive at a high frequency, and wherein in the second operatingstate the third switching element is maintained conductive while thesecond switching element is rendered conductive and non-conductive at ahigh frequency and the first and the fourth switching elements aremaintained non-conductive.
 6. Circuit arrangement as claimed in claim 4,wherein the circuit arrangement is equipped with a power control loopfor controlling the average value of the total power consumed by bothlamps at a desired value by adjusting the time lapse during which thefirst switching element is rendered conductive during each highfrequency period in the first operating state and during which thesecond switching element is rendered conductive during each highfrequency period in the second operating state.